Non-impact printing, and particularly thermal printing, provides advantages in some applications where lack of noise and/or freedom from vibration are important considerations. In certain applications, such as thermal printing of magnetic ink characters, it is particularly important to reproduce accurately and precisely non-horizontal or non-vertical lines, such as angled and curved lines.
For a financial font, such as E13B, for example, horizontal and vertical character bar segments can be printed within appropriate tolerances and parameters, such as a character pitch of eight characters per inch, with each character being formed by a matrix of fourteen dots horizontally and eighteen dots vertically. There are an additional five dots in the horizontal gap between characters.
Typically, the dot matrix character is formed by sequentially moving the thermal printhead past a stationary thermal transfer ribbon, which may be one containing magnetic ink, and a stationary document, while energizing selected thermal printhead resistive elements to effect ink transfer from ribbon to document one character row or column at a time. Thus for a parallel printer, the thermal printhead would be sequentially stepped to eighteen distinct locations and at each location a row of the dot matrix image would be printed. For a serial printer, printing a twelve character amount field, the thermal printhead would be sequentially stepped to nineteen (fourteen character positions plus five between-character spaces) times twelve equals 228 distinct locations, and at each location, a column of up to eighteen dots comprising the image would be printed.
It should be noted that although the foregoing has described an implementation in which the thermal printhead moves, an alternative embodiment, in which the thermal printhead is stationary and the transfer ribbon and paper move, is also viable. This, however, is not a preferred embodiment since it is easier to ensure precise repeatable movements of the thermal printhead, than to rely upon the paper position variations, due to build-up of debris on the paper roller or platen.
For the fourteen by eighteen dot matrix character under consideration, the resolution in the horizontal and vertical directions is set to 0.0065 inches, or one half the minimum character bar thickness. For a font such as E13B, one could obviously improve the printed resolution by increasing the dot density of the thermal printhead. However, this has the practical drawback of increasing the cost of the printhead and the printer, such that the product would no longer be viable in the marketplace. Since the positions of the dots making up the character are fixed, the design of the character is limited by the spacing between dot centers.
It can accordingly be seen that although straight lines may be achieved in the horizontal and vertical directions, it is impossible, with realistic thermal printhead dot density, to achieve an absolutely straight angular line.
Consequently, with prior techniques, the quality of printing of angular lines has not been altogether satisfactory with practical, cost-effective dot matrix thermal technology. As mentioned previously, this is true because of the tradeoff necessary between the expense associated with the dot density of the thermal printhead and the overall speed of the printer, on the one hand, and the desirability for good quality angular lines on the other.